Is it true that a mixture of fuel and oxygen encounters an external heat source for combustion to occur?

The assertion that a mixture of fuel and oxygen encounters an external heat source for combustion to occur is indeed true. Combustion is a chemical reaction that requires three essential components, often referred to as the "fire triangle": heat, fuel, and oxygen.

In order for combustion to start, there must be sufficient heat to raise the temperature of the fuel to its ignition point, which is the temperature at which it starts to combust. The external heat source can come from various stimuli, such as a match, a spark, or another flame. Once this external source ignites the fuel-oxygen mixture, the combustion process can continue as it generates its own heat and may sustain the reaction as long as there is a continuous supply of fuel and oxygen.

Understanding the role of an external heat source is critical in firefighting and fire prevention efforts, as it highlights the importance of controlling sources of ignition in environments where flammable materials are present.